How fast do raindrops fall?

Posted on September 10, 2013 by Weather Guys Editor

The typical speed of a falling raindrop depends on the size of the drop. Gravity pulls everything downward. As an object falls, it experiences a frictional drag that counters the downward force of gravity. When the gravity and frictional drag are balanced, we have an equilibrium fall speed that is known as the terminal velocity of the object. The terminal velocity depends on the size, shape and mass of the raindrop and the density of the air. Thus, it is worth talking a bit about the shape and size of raindrops.

While cartoonists typically draw raindrops in a teardrop or pear shape, raindrops are not shaped in those forms. They are drawn as teardrops to give the image of falling through the atmosphere, which they do.

As the raindrops fall they are flattened and shaped like a hamburger bun by the drag forces of the air they are falling through. Raindrops are at least 0.5 millimeters (or 0.02 inches) in diameter. You will not find a raindrop any bigger than about one-quarter of an inch in diameter; larger than that, the drop will break apart into smaller drops because of the air resistance. Precipitation drops smaller than 0.02 inches in diameter are collectively called drizzle, which is often associated with stratus clouds.

The terminal velocity of cloud droplets, which are typically about 10 microns in radius or 0.0004 inches, is about 1 centimeter per second, or about 0.02 miles per hour. Tiny cloud droplets can stay in the atmosphere because there is upward moving air that overcomes the force of gravity and keeps them suspended in the cloud. Only a very gentle upward movement of air is required to keep them afloat.

Raindrops are larger. A large raindrop, about one-quarter of an inch across or about the size of a house fly, has terminal fall speeds of about 10 meters per second or about 20 mph. That kind of speed can cause compaction and erosion of the soil by their force of impact. Raindrops are of different sizes, and the smaller raindrops are traveling about 2 mph.

Category: Meteorology

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How does this summer compare to last summer?

Posted on September 4, 2013 by Weather Guys Editor

What a difference a year makes!

Despite the recent hot and very humid weather we have had as August ended, this summer has been remarkably mild compared to our real scorcher last summer.

Recall that by the end of August last year, we had recorded 37 days on which the high temperature had been at or above 90 F. This year the grand total is seven (five of them in a row from July 15-19).

Overall, while last summer (June-July-August) averaged 4.73 F above normal, this summer the same period has been only 0.53 F above normal — pretty much a normal summer in terms of temperature.

Most of last summer was very dry, and we ended up 7.1 inches below normal for precipitation. Our incredibly wet spring carried over through June of this year and has resulted in this summer being 3.8 inches above normal for precipitation.

The Climate Prediction Center at the National Weather Service is calling for a warmer than normal September-December for most of Wisconsin coupled with above normal precipitation.

Such seasonal forecasts are based on statistics to a much greater extent than the one- to five-day forecasts commonly portrayed in the print and broadcast media.

As a consequence, one should not hold them to the same expectation for accuracy as the shorter range forecasts.

Last fall was about 0.4 F above normal even though September and October were both slightly below their respective averages.

Despite the great advances in numerical weather prediction on the one- to seven-day range that have been made over the last 25 years, it is still true that only time will tell what kind of autumn we will have.

Category: Seasons

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When was the National Weather Service started?

Posted on August 26, 2013 by Weather Guys Editor

The National Weather Service, or NWS, is a part of National Oceanic and Atmospheric Administration (NOAA). The NWS provides “weather, hydrologic, and climate forecasts and warnings for the United States, its territories, adjacent waters and ocean areas, for the protection of life and property and the enhancement of the national economy.”

The NWS makes and collects surface, marine and atmospheric observations and distributes them nationally and internationally. Professional meteorologists and private forecasting companies often interpret this information provided by the NWS in their weather analysis. In addition to issuing severe weather and marine watches and warnings, the NWS is responsible for computer weather model forecasts, which many forecasters rely on in making their local forecast.

The NWS formed in 1870 through a joint resolution in Congress. It was originally operated by the U.S. Army Signal Corps in the Department of War and made meteorological observations at military stations.

The organization was moved to the Department of Agriculture and renamed the U.S. Weather Bureau in 1891. In 1940 the Weather Bureau became part of the Department of Commerce.

Today the NWS is headed by Dr. Louis Uccellini, a graduate of the University of Wisconsin-Madison, continuing a strong connection between the organization and the state of Wisconsin. The first public storm warning was issued for a Great Lakes storm on Nov. 8, 1870 by Professor Increase Lapham of Milwaukee. On Jan. 3, 1921, UW-Madison’s experimental radio station made the first media weather forecast. Professor Verner Suomi of UW-Madison is known as the Father of Satellite Meteorology; weather satellites are a critical component of the various NWS activities.

Category: Meteorology

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Why are there so few hurricanes every year?

Posted on August 20, 2013 by Weather Guys Editor

Forming over tropical oceans ensures that warm sea-surface temperature (SST) provides a mature hurricane with a means to warm and moisten the air that flows toward the important eye-wall convection. Thus, it is not surprising that hurricanes struggle to develop if the SST is not 79.7 degrees F or warmer. Tropical cyclones also require environments in which the wind speed and direction changes very little with increasing height, in other words, where the vertical wind shear is small.

Certain vast stretches of the tropical ocean have SSTs above the threshold value of 79.7 F and thus qualify as locations where the development of tropical cyclones is favored. However, within such areas it is only when the vertical shear is very low (from the surface to approximately miles above the surface) that hurricanes can form and grow to maturity.

In a given location in the tropics, it is much more likely that the shear condition, not the SST, will vary from one day to the next. There are a number of physical factors that can conspire to render the vertical shear too extreme to allow for hurricane development. One such factor is the presence of the so-called subtropical jet stream that is located between 20 degrees and 30 degrees latitude and about eight miles above the ground in both hemispheres. The subtropical jet stream is an ever-present feature of the general circulation of the tropics and has wind speeds routinely in excess of 130 mph.

Such strong winds well above the surface are more than sufficient to provide a toxic amount of vertical shear to a nascent tropical cyclone. The small number of hurricanes every year testifies to the hostility of the environment to their development.

Category: Tropical

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Why do hurricanes form over the tropical oceans?

Posted on August 13, 2013 by Weather Guys Editor

The organized storms we experience here in Madison in fall and winter are known as mid-latitude cyclones. One of the most notable characteristics of these storms is the presence of strong temperature, humidity and wind contrasts at what are known as fronts. In fact, fronts are such an integral part of the structure of these storms that they are often referred to as frontal cyclones.

Unlike the mid-latitude cyclones, the air temperature is fairly uniform throughout a hurricane. This is the case even though the air sinks and is warmed by compression on the outer edge of the hurricane, while near its eyewall the air rises vigorously and cools by expansion.

Even though some of that cooling is counteracted by the enormous amount of latent heat release that occurs in the eyewall convection as water vapor is condensed into liquid water, one might still reasonably expect the eyewall region to be cooler than the periphery of the storm. This is not the case because the low-level air that is directed toward the center of the storm is greatly modified during that journey — making constant contact with a wildly windswept and very warm ocean surface.

As a consequence of the warm sea-surface temperature (SST), the inflow air is both warmed and moistened as it flows toward the eyewall — evaporation from the ocean being enhanced by the strong winds and disturbed ocean surface. For this reason, very warm, very moist air is the optimal fuel for the sustenance of a hurricane. This explains why warm, tropical ocean basins are where hurricanes form.

Category: Tropical

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